1. Field of the Invention
This invention relates generally to a power supply circuit, and more particularly to a novel power supply circuit having a circuit limiting the duty ratio of a pulse signal to be supplied to a switching element.
2. Description of the Prior Art
In a known stabilized power supply circuit, a commercial AC voltage is fed through a power plug and a power switch to a rectifier circuit and converted to a DC voltage. This DC voltage is applied to a series connection of the primary winding of a transformer and a switching transistor which is supplied with a PWM (pulse width modulated) pulse signal from a PWM modulator to be switched ON and OFF. Thus, an AC voltage is induced across the secondary winding of the transformer and then fed to a rectifying circuit to be rectified as a DC voltage. This DC voltage is delivered to an output terminal.
In this case, the DC voltage delivered to the output terminal is detected by a detecting circuit whose detected output is supplied through a coupler for insulation and separation such as a photo-coupler to a modulation circuit as its modulating signal. This modulation circuit is supplied with a pulse signal from an oscillating circuit as a carrier. Thus, the pulse width of the PWM pulse from the modulation circuit is changed in accordance with the voltage at the output terminal, and thus the DC voltage at the output terminal is stabilized at a constant value.
In this case, the modulation circuit produces, for example, a triangular wave signal S.sub.t based upon the pulse signal from the oscillating circuit. The signal S.sub.t and the signal S.sub.d from the detecting circuit are level-compared and hence a PWM pulse P.sub.m is produced which is "1" when S.sub.t .gtoreq. S.sub.d and "0" when S.sub.t &lt; S.sub.d.
When a load connected to the output terminal becomes heavy, the signal S.sub.d reaches a very low level. As a result, at this time the duty ratio of the pulse P.sub.m exceeds a set value (which is usually about 70.about.80%) and hence the modulation becomes 100% at this time (over-modulation).
With the above prior art power supply circuit, during the ON-period of the switching transistor energy is stored in the primary winding of the transformer and is derived from the secondary winding of the transformer during the OFF-period of the switching transistor. Thus, when the duty ratio of the pulse P.sub.m exceeds the set value, a current is fed to the primary winding before an output current from the secondary winding becomes zero during the OFF-period of the switching transistor. As a result, there occurs a fear that the core of the transformer is saturated and then damaged, which would cause the transformer to be damaged or the rectifying circuit to be damaged also.
Further, there may appear a case where an output voltage V.sub.d of the rectifying circuit contains fluctuation components such as ripple components. In such a case, if a tolerance value (maximum value) of the duty ratio of the pulse P.sub.m is set for the minimum value of the voltage V.sub.d, at the maximum value of the voltage V.sub.d a current greater than at the minimum value of the voltage V.sub.d will flow to the primary winding, so that the transformer, transistor or rectifying circuit is damaged due to the same reason as that set forth above.